They are led by a Sergeant, who is usually a member of a Tactical Squad that chooses to impart his wisdom to a younger generation, but may also be a Veteran Scout who refused the opportunity to become a full-fledged Space Marine. In the case of the Blood Ravens, they are led by Steve Blum voicing an angsty, ineffectual dork, who he hopes one day to teach all the Blood Ravens to be angsty, ineffectual dorks. His most important lesson is to go out of the way to point out problems and question every solution as if it was the most inexperienced idea you've ever heard.

Also, if you even say the word "bonk" within a hundred feet of an Angry Marine Scout with a Power Bat, he will beat you so hard you won't know which end of your sternum you're looking through.

Back in the times of the Great Crusade however, the Space Marine Legions didn't have Scouts at all. Instead they had the even better Legion Reconnaissance Squads. Instead of being made out of simple Neophytes that were to experience their tour of actual combat, Legion Reconnaissance Squads were made out of experts at reconnaissance, intel gathering, guerilla warfare, assassinations and sabotage. If they were fighting along their brethren on the battlefield, then they'd become flankers and wreck havoc and confusion among the enemy's ranks.

So how mighty is the Scout, as Junior Marine, on a stat line basis? Well, he has always gotten slightly less good armour, due to not power armouring. In codex Imperialis and Codex Ultramarines, Scouts had M4 WS4 BS3 S4 T3 W1 I4 A1 Ld7, which made then as good at punching as Space Marines (assuming 4 all besides W and A was the space marine), but not as good at taking punches or shooting shit. In 5th edition, Scouts traded their Weapon Skill for their Toughness, and probably got a buff to leadership. Making them guardsmen when it comes to hitting things but space marines in the wounding. By 6th edition, all scouts/claws had space marine muscles, but had the finesse of a guardsman. When the Space Wolf and Blood Angel codices continued the trend, everyone expected this to hold through the edition. Then the Space Marine Codex dropped. At no point increase on anything, scouts and anything scout related got WS/BS 4. The Dark Angel codex dropped as well, with the same stat line boost as the Space Marine codex, and free access to Grim Resolve (i.e. Chapter Tactics for Dark Angels). As of current with all 7th edition codex released, half of the Adeptus Astartes books have junior Marines with WS/BS 4. This may be fair for Space Wolves, because blood and sky claws get Rage. It's still full of derp, as while the matching S and T make sense (the black carapace they lack would let them keep abilities like Move Through Cover in Power Armor if Gee Dubs bothered implementing it), by definition they have way less experience than non-newb Marines and should be worse at getting shit done.

All of this is of course forgetting that in 4th Edition Scouts had WS/BS of 4 to go along with S/T 4, so if anything 5th Edition was a nerf and 7th restored them to their rightful place of being useful. To go along with the fact that a WS/BS of 4 actually makes sense for a group that doesn't get to stop being Scouts until a slot opens up in a Company.

Space Wolves Wolf Scout: Scouts in the Space Wolves are bitter, experienced veterans taken from the Grey Hunters (the Space Wolves equivalent of the Tactical Marines); most of them are introverts that find it hard to fit in with the rest of the (incredibly gregarious) Space Wolves and prefer to operate more-or-less alone. New recruits are put in Blood Claw squads instead.

Black Templars Neophytes: The Black Templars are too cool to use Scouts. Instead, when an Initiate (the Black Templar equivalent of a Tactical Marine) decides he has learned enough, he takes on a Marine-in-training, referred to as Neophytes, which are then mixed into Crusader squads. This is nominally to pass the experience of an Initiate to a Neophyte, but in reality, the Neophytes just make amazing meatshields. And weapons caddies.